The OSA applies to all of the UK.

England is the only one of the four without its own parliament.

Providing you accurately define the Irish in question all four are subject to the OSA, none have actively opposed it in any meaningful way.

They do, but with limited powers. It’s not exactly the same as US states and the federal government, but you can think of them as regional governments or levels of government in that sort of way. The Scottish Parliament can pass some types of laws for Scotland. But Westminster (usually) passes laws for the entire UK.